Take a look at the threads on licensing. In general/summary, if it's pre 1960 you can drive it unladen for fun, but if you want to carry things, use it for work, or drive a later vehicle then you'll need to down rate the vehicle to C1 (approximately a luton van) or take your C licence.
Depending on what you want to do with the box body, registering as a motorhome might be another option to explore.